Staplers and related stapling devices eject staples out from a loading track into a work piece. Staplers are commonly known in two general forms or applications. A staple gun inserts a staple substantially unchanged into a work piece such as wood. A desktop stapler presses a staple against an anvil whereby the staple legs are bent behind sheets of paper. In either type of design, it is possible to eject a staple unintentionally toward a user by pointing the staple ejection port, for example, toward the face. Some staplers and most staple guns use energy stored in a spring to eject the staple. With spring-powered staplers being so common, such an injury to the user is possible.
Another type of stapler uses an electric, air, combustion, or other non-manual power source. Safety interlocks are well known in such powered applications. Typically a safety button near the staple ejection area must be pressed to enable a power circuit to be energized. In effect the trigger can switch the power device only when the tool is pressed against a work piece. U.S. Pat. No. 5,715,982 (Adachi) is an example of a protruding safety button used to lock out a trigger action in a power tool.
A similar type safety mechanism has not been effectively implemented in a manual stapler. This is likely because it is simple to disable a power circuit. For example, a microswitch may be linked to the safety button whereby the switch remains open until the button is pressed. This operation requires little force and allows for a relatively low strength plastic button assembly that is easily pressed inward to enable use. However, it is more complicated to disable a manually actuated device. In the case of a spring-actuated stapler the large impact forces make a disabling system especially difficult. One reason is that the stored potential energy of the spring is difficult to de-energize or redirect without using complicated or bulky latches, blocks, stops, etc. Also, a catch or safety that blocks the spring action must be rather bulky to absorb the high impact energy from the spring.
Moreover, a staple or other fastener may be ejected from a fastening device under different conditions. If a work piece is present, the energy of the staple is absorbed as the staple penetrates or bends about the material. If no work piece is present the staple is “dry fired.” Most of the energy of the moving striker is dissipated by an internal absorber inside the conventional stapler. With a lightweight work piece the stapling energy is absorbed partly by the work piece and partly by the absorber.
The case of a dry fire is a concern of the present invention, where there is no work piece to stop the staple. Although most of the stapling energy is dissipated in the absorber, the staple will continue to shoot out under its own momentum. These and other issues are addressed by the present invention.